The present invention relates generally to plastic injection molding systems and more particularly to gas-assisted plastic injection molding systems and processes.
There are numerous known systems for plastic injection molding. In conventional plastic injection molding systems, plastic pellets are melted in an injection molding machine and advanced by a screw ram into a mold cavity. The mold cavity is formed between two mold halves (a stationary core member and a moveable cavity member) typically through one or more sprue bushings, a manifold and/or a hot runner system.
The two halves of the mold are clamped together and the plastic is injected into the mold cavity. The molten plastic material in the cavity is allowed to cool and harden in the cavity, typically by a cooling system which circulates a cooling fluid through one or more of the mold members. When the part is sufficiently hardened, the mold is opened and the part removed, typically by use of one or more ejector pins.
Some of the known systems utilize a gas in the injection molding process and are commonly known as “gas-assisted injection molding systems.” In these systems, the gas is injected into the molten plastic material through the plastic injection nozzle itself or through one or more pin mechanisms strategically positioned in the mold, sprue bushings, manifolds, or hot runner systems. It is also possible to inject the gas directly into the molten plastic in the barrel of the injection molding machine. The gas, which typically is an inert gas, such as nitrogen, is injected under pressure and forms one or more hollow cavities or channels in the molded part. The benefits of gas-assisted injection molding processes are well-known, and include the cost savings through the sue of less plastic material, producing parts which are lighter in weight, and producing parts which have better surface definitions and finishes. Also, the hollow parts have thinner wall members and thus cool in a faster manner, thereby increasing production output.
Another plastic injection molding system which utilizes gas injects the gas into the mold cavity along one or more exterior surfaces of the molded part. The pressurized gas forces the plastic against the opposite surface or surfaces of the mold cavity and forms a part with superior surface characteristics on the appearance surfaces. On the non-appearance or “backside” of the part, this system and process typically leaves numerous visual defects.
It is also known to create a laminate product in which a piece of fabric or the like is positioned in the mold and the plastic material is injected or formed on the fabric. This process typically uses a structural foam as the plastic material.
Although many of these plastic injection molding systems and processes operate satisfactorily and have produced commercially acceptable plastic injection molded parts and components, including laminate-type components, there is a need for improved systems and processes, particularly those which can produce a superior product, a less costly product, or a product which can be manufactured in a faster and more efficient manner.